Chung Wung Bark

Characteristics of the Dye-Sensitized Solar Cells Using TiO2 Nanotubes Treated with TiCl4

The replacement of oxide semiconducting TiO2 nano particles with one dimensional TiO2 nanotubes (TNTs) has been used for improving the electron transport in the dye-sensitized solar cells (DSSCs). Although use of one dimensional structure provides the enhanced photoelectrical performance, it tends to reduce the adsorption of dye on the TiO2 surface due to decrease of surface area. To overcome this problem, we investigate the effects of TiCl4 treatment on DSSCs which were constructed with composite films made of TiO2 nanoparticles and TNTs. To find optimum condition of TNTs concentration in TiO2 composites film, series of DSSCs with different TNTs concentration were made. In this optimum condition (DSSCs with 10 wt% of TNT), the effects of post treatment are compared for different TiCl4 concentrations. The results show that the DSSCs using a TiCl4 (90 mM) post treatment shows a maximum conversion efficiency of 7.83% due to effective electron transport and enhanced adsorption of dye on TiO2 surface.

Template-engineered epitaxial BiVO4 photoanodes for efficient solar water splitting

Bismuth vanadate (BiVO4) has attracted significant attention as a promising photoanode material for hydrogen production via photoelectrochemical (PEC) water splitting because of its narrow optical band gap and suitable band edge positions for water oxidation. However, the actual photoactivity of BiVO4 is considerably limited by its poor electron transport and slow water oxidation kinetics. Although several studies have been carried out to improve its photo-efficiency via the enhancement of electron transport and water oxidation kinetics, only a few studies have reported the growth of epitaxial BiVO4 to explore the fundamental properties of BiVO4 for PEC water splitting because extremely flat epitaxial films exhibit poor photo-efficiency because of their low surface-active area. However, studies of epitaxial BiVO4 still have the potential to provide new routes for improving its photo-efficiency. In this study, the growth of epitaxial BiVO4 is investigated using a thin γ-WO3 template layer deposited on a SrTiO3(001) substrate covered by a SrRuO3 (SRO) bottom electrode using pulsed laser deposition. Consequently, at 1.23 V vs. the RHE (reversible hydrogen electrode), the photocurrent density of epitaxial BiVO4 on the γ-WO3 template layer (2.20 mA cm−2) is approximately 10 times that of bare BiVO4, related to the effective charge transfer by the γ-WO3 intermediate layers and the subsequent increase in the surface-active area of epitaxial BiVO4. These results strongly suggest that epitaxial BiVO4 grown using a template layer can be a cornerstone for the in-depth understanding of the fundamental properties of BiVO4 for PEC water splitting.

Characteristics of AZO/Cu/AZO Multilayer Thin Films Prepared on Polyethersulfone Substrate at Room Temperature

Highly conducting Al doped ZnO(AZO)/Cu/AZO triple multilayer thin films were deposited on polyethersulfonesusbtrate at room temperature. We investigated the structural, electrical, optical, and mechanical properties of triple multilayers as a function thickness of Cu layers. The triple multilayer with flexible substrate had advantages such as low sheet resistance and stable mechanical properties as compared with single oxide layer. From the results, sheet resistance value of AZO(50 nm)/Cu(9 nm)/AZO(50 nm) multilayer was 12 Ω/□, and average optical transmittance(380–770 nm) value of multilayer was 80%. Moreover the triple multilayer showed mechanical flexural strength properties than single-layered AZO thin films during bending test due to the existence of ductile Cu metal layer.

Large enhancement of the photovoltaic effect in ferroelectric complex oxides through bandgap reduction

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.

Synthesis and Characterization of WO3 Doped TiO2 Particle/Nanowire Layer in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have been intensively studied since their discovery; however, a number of problems remain to be solved in order to enhance their efficiency. In particular, one of the main limiting factors is the electron recombination that occurs due to contact between the transparent conductive oxide and the redox electrolyte. In this work, we report an improvement in the photovoltaic characteristics of dye-sensitized solar cells by using mixtures of TiO2, TiO2 nanowires (TNWs), and WO3. DSSCs were constructed by application of WO3 mixed with a composite of varying percentages of TiO2 nanoparticle and TNWs. The addition of WO3 reduces the surface trap states of TiO2, improving its power conversion efficiency by suppressing charge recombination, and increasing the driving force of electron injection. A DSSC fabricated using a TiO2–WO3 mixture was found to have a maximum conversion efficiency of 5.47% owing to the effective prevention of electron recombination. As a result, DSSCs based on TiO2–WO3 mixtures exhibited better photovoltaic performance compared to cells fabricated from pure TiO2

Influence of Fe2O3 Doping on TiO2 Electrode for Enhancement Photovoltaic Efficiency of Dye-Sensitized Solar Cells

In this work, we report for the first time the improvement of the photovoltaic characteristics of dye-sensitized solar cells (DSSCs) by doping TiO2 with Fe2O3. DSSCs were fabricated using various percentages of Fe2O3-doped TiO2 composite nanoparticles. The Fe2O3-doped DSSCs exhibited a maximum conversion efficiency of 5.76% because of the effective electron transport. DSSCs based on Fe2O3-doped TiO2 films showed better photovoltaic performance than cells fabricated with only TiO2 nanoparticles. This result was attributed to the prevention of recombination between electrons in the TiO2 conduction band with the dye or electrolytes. A mechanism was suggested based on impedance results, which indicated improved electron transport at the interface of the TiO2/dye/electrolyte.

Structural and optical properties of bandgap engineered bismuth titanate by cobalt doping

The wide band gap of complex oxides is one of the major obstacles limiting their use in photovoltaic cells. To tune the bandgap of complex oxides, nano-sized bismuth titanate-based powders were synthesized by conventional solid reaction method. X-ray diffraction patterns confirmed that all powders were crystallized in an orthorhombic structure. The photoluminescence signal shows that there was no contribution to the optical bandgap from unwanted oxygen vacancy. The UV-vis absorption spectra of LaCo-BiT powder showed that the optical bandgap drastically decreased from 3.1eV to 2.5eV, while those of of BiT and La-BiT showed change in the optical bandgap. From these observations, we could experimentally confirm that cobalt atoms were responsible for the modification of the electronic structure in BiT-based oxides. This approach to controlling the bandgap could be applied to other complex oxides materials, such as other types of Aurivillius phase materials for use in emerging oxide optoelectronic and energy applications.

The effect of dye-sensitized solar cell based on the composite layer by anodic TiO2 nanotubes

TiO2 nanotube arrays are very attractive for dye-sensitized solar cells (DSSCs) owing to their superior charge percolation and slower charge recombination. Highly ordered, vertically aligned TiO2 nanotube arrays have been fabricated by a three-step anodization process. Although the use of a one-dimensional structure provides an enhanced photoelectrical performance, the smaller surface area reduces the adsorption of dye on the TiO2 surface. To overcome this problem, we investigated the effect of DSSCs constructed with a multilayer photoelectrode made of TiO2 nanoparticles and TiO2 nanotube arrays. We fabricated the novel multilayer photoelectrode via a layer-by-layer assembly process and thoroughly investigated the effect of various structures on the sample efficiency. The DSSC with a four-layer photoelectrode exhibited a maximum conversion efficiency of 7.22% because of effective electron transport and enhanced adsorption of dye on the TiO2 surface.

The Effect of Phosphor-TiO2 Layer on the Performance of Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are composed of an electrode made of a dye-adsorbed nanoporous TiO2 layer on a fluorine-doped tin oxide (FTO) glass substrate, redox electrolytes, and a counter electrode. In this study, phosphor is introduced into the TiO2-layer electrode of a DSSC. The admixed phosphor content in the TiO2 paste is varied from 1.0 to 10.0 wt%. By a conversion-luminescence process, ZnGa2O4:Mn2+ phosphor improves light harvesting and increases the photocurrent. The phosphor elevates both, the energy level of electrons in the oxide film and Voc of the DSSC. Using a TiO2 electrode containing 5.00 wt% of admixed ZnGa2O4:Mn2+, the light-to-electricity energy-conversion efficiency of the DSSC reaches 8.02%, which is higher by a factor of 1.25 than that of a DSSC without ZnGa2O4:Mn2+.

Effect of Post Annealing in Various Atmospheric Environment Applied to ZnO:Ga Films

In this study, transparent conductive films (TCO) of ZnO:Ga (GZO) were deposited by facing target sputtering to explore the effect of post-annealing on the structural, electrical and optical properties of the films. As deposited films have been annealed to each different temperature condition in various atmosphere environments (air, N2, Vacuum N2). In the result, in air atmosphere condition, ZnO:Ga films have lost their characteristics as TCO films, because rapid oxidation occurred over 300°C. But in other atmosphere condition, contacting with oxygen is reduced more, as rapid rise of resistivity has been prevented.